2.2. Disk Configuration
The first subsection below describes the disk configuration issues that must be considered when planning a Linux FailSafe system. It explains the basic configurations of shared and non-shared disks and how they are reconfigured by Linux FailSafe after a failover. The second subsection explains how disk configurations are specified when you configure the Linux FailSafe system.
2.2.1. Planning Disk Configuration
For each disk in a Linux FailSafe cluster, you must choose whether to make it a shared disk, which enables it to be failed over, or a non-shared disk. Non-shared disks are not failed over.
The nodes in a Linux FailSafe cluster must follow these requirements:
The system disk must be a non-shared disk.
The Linux FailSafe software, in particular the directories /var/run/failsafe and /var/lib/failsafe, must be on a non-shared disk.
Choosing to make a disk shared or non-shared depends on the needs of the highly available services that use the disk. Each highly available service has requirements about the location of data associated with the service:
Some data must be placed on non-shared disks
Some data must be placed on shared disks
Some data can be on shared or non-shared disks
The figures in the remainder of this section show the basic disk configurations on Linux FailSafe clusters before failover. Each figure also shows the configuration after failover. The basic disk configurations are these:
A non-shared disk on each node
Multiple shared disks contained Web server and NFS file server documents
In each of the before and after failover diagrams, just one or two disks are shown. In fact, many disks could be connected in the same way as each disk shown. Thus each disk shown can represent a set of disks.
A Linux cluster can contain a combination of the basic disk configurations listed above.
Figure 2-1 shows two nodes in a Linux FailSafe cluster, each of which has a non-shared disk with two resource groups. When non-shared disks are used by highly available applications, the data required by those applications must be duplicated on non-shared disks on both nodes. When a failover occurs, IP aliases fail over. The data that was originally available on the failed node is still available from the replacement node by using the IP alias to access it.
The configuration in Figure 2-1 contains two resource groups, Group1 and Group2. Group1 contains resource 192.26.50.1 of IP_address resource type. Group2 contains resource 192.26.50.2 of IP_address resource type.
Figure 2-2 shows a two-node configuration with one resource group, Group1. Resource group Group1 has a failover domain of (xfs-ha1, xfs-ha2). Resource group Group1 contains three resources: resource 192.26.50.1 of resource type IP_address, resource /shared of resource type filesystem, and resource shared_vol of resource type volume.
In this configuration, the resource group Group1 has a primary node, which is the node that accesses the disk prior to a failover. It is shown by a solid line connection. The backup node, which accesses the disk after a failover, is shown by a dotted line. Thus, the disk is shared between the nodes. In an active/backup configuration, all resource groups have the same primary node. The backup node does not run any highly available resource groups until a failover occurs.
Figure 2-3, shows two shared disks in a two-node cluster with two resource groups, Group1 and Group2. Resource group Group1 contains the following resources:
Resource 192.26.50.1 of type IP_address
Resource shared1_vol of type volume
Resource /shared1 of type filesystem
Resource group Group1 has a failover domain of (xfs-ha1, xfs-ha2).
Resource group Group2 contains the following resources:
Resource 192.26.50.2 of type IP_address
Resource shared2_vol of type volume
Resource /shared2 of type filesystem
Resource group Group2 has a failover domain of (xfs-ha2, xfs-ha1).
In this configuration, each node serves as a primary node for one resource group. The solid line connections show the connection to the primary node prior to failover. The dotted lines show the connections to the backup nodes. After a failover, the surviving node has all the resource groups.
Other sections in this chapter provide more specific information about choosing between shared and non-shared disks for various types of data associated with each highly available service.
2.2.2. Configuration Parameters for Disks
There are no configuration parameters associated with non-shared disks. They are not specified when you configure a Linux FailSafe system. Only shared disks (actually, the logical volumes/partitions on shared disks) are specified at configuration. See the Section 2.3.1 for details.